1. Field
The invention is in the field of blasting blocks of rock from larger rock formations or larger blocks such as the blasting of loaves and blocks of rock in quarries, and in the field of blasting openings in rock.
2. State of the Art
In quarries, and particularly dimension stone quarries such as granite, marble, and limestone quarries, it is usual practice to break large pieces of rock, generally referred to as loaves, from the solid rock formation, and then, either at the site, or at a different site to which the loaves are moved, break the loaves into smaller blocks that are then further cut and finished to provide the finished rock product. The loaves are generally up to about fifty feet in length by about twenty feet in width by about fifteen feet in height and weigh in the range of about fifteen thousand tons. The smaller blocks into which the loaves are broken typically are about six feet in width by about five feet in height by about six to eight feet in length.
In separating the loaves from the solid rock formation, cuts in the rock to define the ends of the loaves are generally made by water jet cutting or by burning or spalling. Once the ends are cut to define the length of the loaf and to provide relief, bore holes are drilled into the rock along the desired lines of breakage at the back of the loaf and the bottom of the loaf between the end cuts. These bore holes are then filled with an explosive, the explosive is detonated, and the detonation separates the loaf of rock from the solid rock formation. Depending upon the explosives used, the holes along a horizontal break line (the bottom of the loaf) may be drilled at a slight angle in order to hold a liquid. This angle is normally about 2.degree. but may range up to about 60.degree.. The loaves are then further broken into the smaller blocks. These smaller blocks are moved to the finishing facility and cut and finished. Current practice in forming the smaller blocks at granite and similar quarries is to drill the loaves along further desired break lines. Holes about 11/4 inches in diameter are drilled about every 51/2 inches along the desired break lines. The holes are drilled almost through the block, but not all of the way through so the holes will hold water therein.
In blasting the smaller blocks, small explosive power is all that is needed. Such power can be provided by a length of detonating cord inserted into the hole and extending the entire length of the hole. However, the standard hole drilled in the rock is about 1-11/4 inch in diameter. Explosives filling such a hole would be much too powerful to provide the desired breaks and would result in significant overbreakage causing the rock to split where not desired and unwanted cracks to form extending peripherally around the diameter of the blast hole rendering much of the rock damaged, unusable, and wasted. The detonating cord, however, is significantly smaller in diameter than the diameter of the hole. If the detonating cord is merely placed in the hole and detonated, most of the explosive power is absorbed by the air in the empty hole and the blast is not effective to crack and break the rock along the desired break line. In some cases, if detonating cord of relatively large explosive power is used, it can merely be placed in the hole in air and detonated. However, this uses much more explosive power than necessary. Current practice in many quarries is to place the detonating cord in the hole, fill the hole with water, and then detonate the cord. The water transmits the explosive force of the detonation hydrostatically to the rock and the rock is broken as desired. However, the force of the explosion usually causes many small cracks, called spider cracks, to form extending from the hole anywhere from three inches to one foot into the rock. In finishing the block of rock, it is necessary to cut off the rock having the spider cracks. This wastes a substantial amount of the rock which is an economic loss to the quarry. For example, if the spider cracks extend three inches into the rock on all sides so that three inches is cut off all sides of a five by six by eight foot block, this wastes almost twelve percent of the rock in the block. If cracks extend twelve inches into the block so twelve inches of rock has to be cut, over forty percent of the block is wasted.
Where high quality stone is being broken into blocks and the excessive waste due to spider cracks is to be avoided, the blocks can be broken by wedging. In wedging, steel wedges are manually driven into the bore holes along the desired break line to force the block to break and separate from the loaf. However, the wedging is labor intensive and time consuming so use of explosives for breaking the block is generally preferred.
Attempts have been made to reduce the spider cracks in the rock blocks by arranging the detonating cord in the bore hole so that it extends down one side of the bore hole and up the other against the rock to be broken along the desired break line and the hole is then filled with sand rather than water. However, for the sand to be effective and to effectively flow into and fill a 1-11/4 inch diameter bore hole, the sand has to be very dry. It is extremely difficult to keep sand as dry as necessary in a quarry and even dry sand is difficult to pour into the hole. Generally dry sand is about forty percent void space. While this procedure appears to reduce cracking to some extent it is not as effective as desired and, in addition to the difficulty in filling the holes with sand, it is difficult and time consuming to correctly line-up the detonating cord and to provide the separations necessary to keep the cord apart and correctly aligned.
In many cases, in addition to breaking loaves into smaller blocks, the detonating cord with surrounding water provides enough explosive power for breaking the large loaves of rock from the solid rock formation so the water and detonating cord can be used in those situations also. However, there is still the problem of spider cracks that have to be removed before the rock is ready for finishing.